Prior art has recognized that for certain lift truck operating conditions, the truck may become unstable, and various systems for limiting or curtailing various lift truck operations have been used in the past. In many cases lift truck travel speed has been decreased to an unnecessarily low value, decreasing productivity more than necessary.
In a preferred form of the invention involving a rider-controlled vehicle, a plurality of vehicle conditions are sensed, and in response to those conditions a value representing a travel speed at which the vehicle can be driven in an instantaneously stable condition is derived. That value may be called the "instantaneous speed limit", and the signal representative thereof may be called the "instantaneous speed limit signal". That signal, together with the speed and acceleration desired by the rider, or operator, is connected to the vehicle traction motor control system so that vehicle speed ordinarily cannot exceed the speed represented by the instantaneous speed limit signal. The operator ordinarily will indicate a desire for an increase or a decrease in speed by his manual positioning of a control handle. According to the invention, vehicle travel may occur at any speed less than that represented by the instantaneous speed limit, or even at zero speed, if such a lesser speed is requested by the operator input signal. With the truck proceeding at the speed commensurate with the instantaneous speed limit signal, a sensed vehicle condition may change so as to reduce the magnitude of the instantaneous speed limit signal. The load elevation might increase, for example. Under such circumstances, the instantaneous speed limit signal will momentarily be less than a signal representing the truck travel speed. In a preferred embodiment of the invention, the occurrence of such a condition results in the reduction of tractive effort, but in no decelerating force other than that provided by motor, gearing, and tire friction, so the vehicle will coast at a descending speed until vehicle speed reaches the new instantaneous speed limit. In some applications of the invention, it may be deemed desirable to apply some additional decelerating force.